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Aircraft antennas



 
 
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  #1  
Old October 23rd 06, 04:12 PM posted to rec.aviation.homebuilt
ccwillwerth
external usenet poster
 
Posts: 8
Default Aircraft antennas

Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Charlie


  #2  
Old October 23rd 06, 04:35 PM posted to rec.aviation.homebuilt
Jim Carriere
external usenet poster
 
Posts: 57
Default Aircraft antennas

ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?


Please excuse my "piggybacking" your question, can anyone with an
informed opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea
at first glance...
  #3  
Old October 23rd 06, 06:34 PM posted to rec.aviation.homebuilt
Orval Fairbairn
external usenet poster
 
Posts: 824
Default Aircraft antennas

In article ,
"ccwillwerth" wrote:

Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Charlie


The ground plane should extend an antenna length around the antenna base.

Copper or aluminum tape will serve the purpose very well. This is
outlined in CAM 18. Make sure that your ground has a good electrical
connection with the elements of the ground plane.
  #4  
Old October 24th 06, 12:27 AM posted to rec.aviation.homebuilt
Stuart & Kathryn Fields
external usenet poster
 
Posts: 328
Default Aircraft antennas

Charlie: I recently installed a VHF com antenna on the tailboom frame of my
Baby Belle helo. It's dimensions are 16" X 12" Which are less than the
length of the antenna. I ran an antenna check using an MFJ 250 antenna
analyzer and the highest VSWR I got was 2.2. This was so much better than
the bent antenna that had been on the belly of the ship which did have a
large ground plane but had VSWR greater than 3 and in some cases as high as
5. One thing you can do is try out a prototype ground plane and then have a
Ham or someone with the right equipment do a check.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"ccwillwerth" wrote in message
...
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe
so that it would be just under the fabric. The antenna is the type that
has a ceramic insulator on the bottom of the antenna that insulates the
stainless steel antenna from the airframe. Does the antenna need a large
plate for a ground plane or will a small plate be sufficient? If a ground
plane is required, can the copper foil tape be used on the inside of the
fabric as the ground plane?

Charlie



  #5  
Old October 24th 06, 02:53 AM posted to rec.aviation.homebuilt
Stuart & Kathryn Fields
external usenet poster
 
Posts: 328
Default Aircraft antennas

Oops. The installed ground plane dimensions are 16"X12".
--
Kathy Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478
(760) 408-9747 general and layout cell
(760) 608-1299 technical and advertising cell

www.vkss.com
www.experimentalhelo.com


"Stuart & Kathryn Fields" wrote in message
.. .
Charlie: I recently installed a VHF com antenna on the tailboom frame of

my
Baby Belle helo. It's dimensions are 16" X 12" Which are less than the
length of the antenna. I ran an antenna check using an MFJ 250 antenna
analyzer and the highest VSWR I got was 2.2. This was so much better than
the bent antenna that had been on the belly of the ship which did have a
large ground plane but had VSWR greater than 3 and in some cases as high

as
5. One thing you can do is try out a prototype ground plane and then have

a
Ham or someone with the right equipment do a check.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"ccwillwerth" wrote in message
...
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe
so that it would be just under the fabric. The antenna is the type that
has a ceramic insulator on the bottom of the antenna that insulates the
stainless steel antenna from the airframe. Does the antenna need a

large
plate for a ground plane or will a small plate be sufficient? If a

ground
plane is required, can the copper foil tape be used on the inside of the
fabric as the ground plane?

Charlie





  #6  
Old October 24th 06, 03:28 AM posted to rec.aviation.homebuilt
[email protected]
external usenet poster
 
Posts: 9
Default Aircraft antennas


Orval Fairbairn wrote:
In article ,
"ccwillwerth" wrote:

Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe.


Charlie, you will get several opinions, but here is mine.

If at all possible, get a "broadband" VHF Com antenna, not a wire whip.
The "broadband" fiberglass antennas have a VSWR of less than 2:1 across
the range of 118 to 136MHz, while the metallic whip will have an
bandwidth of only about 5Mhz where the VSWR is below 2:1. At the band
edges, the VSWR will be high enough to cause the VSWR protection
circuitry in transistorized transmitter to shut the output power down
to nearly zero. Although the wire-whip can be cut&tuned to just Unicom
frequencies (122.7 to 123.6 Mhz), it could be marginal for transmission
at some ATC frequencies. The whip will work ok for receiving even at
the band-edges, because the receiver doesn't care about the VSWR.

If your fuselage is anything like my Piper PA20, there are enough metal
tubes to act as a ground plane without adding any additional conductive
material, other than a mounting plate. I would put the antenna base on
a metal plate which is just below the plane formed by the fabric.
Radius the edges of the plate so that the fabric doesn't ride on a
sharp edge. The plate could be long enough to bridge between two
fuselage cross-brace tubes, but only about 4 to 6" wide. It must be
electrically "bonded" to the cross-braces, so to avoid drilling holes
in the cross-braces, your idea of welding some attach "ears" to the
cross-braces to mount the plate is good.

  #7  
Old October 24th 06, 03:55 AM posted to rec.aviation.homebuilt
[email protected]
external usenet poster
 
Posts: 9
Default Aircraft antennas


Jim Carriere wrote:

Please excuse my "piggybacking" your question, can anyone with an
informed opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea
at first glance...


VHF COM, NAV, and ELT antennas placed inside a welded-tube fuselage
aren't worth crap!
What is the biggest complaint of anybody trying to use a Air-band VHF
hand-held transceiver inside the cabin of a metal aircraft? They cant
be heard! They all end up putting an external antenna on their
aircraft. And no, it is not just the inefficiency of the rubber-ducky
that causes this.

The fundemantal problem is that the wavelength at 120Mhz is 2.5 meters.
In order for a radio wave to pass through an opening in a metallic
structure, the dimensions of the opening need to approach a half
wavelength in diameter, and even then the wave is greatly attenuated.
Just to make the point, a half-wave at 120Mhz is 49". How many of the
openings whose edges are defined by the fuselage longerons, the
cross-brace and diagonal brace tubes are 49" across? Answer, none of
them. Even the windscreen opening is usually not that large.
A welded tube fuselage makes a real good "screen room"... Do yourself a
favor, and put the antennas on the outside where they belong...

  #8  
Old October 24th 06, 11:53 AM posted to rec.aviation.homebuilt
XCOM
external usenet poster
 
Posts: 1
Default Aircraft antennas

ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Charlie



Our experience is that anything inside a structure especially metal is
NOT going to work. As one other said... put the aerial on the outside
where it's meant to go.

Also we have just added a table on VSWR effects and power drop off with
a bad aerial, check http://www.mcp.com.au/xcom760/faq/faq.html for
details... right down the bottom of the page (All the way down)

Interesting if you have a bad aerial Regards Michael from XCOM
  #9  
Old October 24th 06, 06:59 PM posted to rec.aviation.homebuilt
RST Engineering
external usenet poster
 
Posts: 1,147
Default Aircraft antennas


"Jim Carriere" wrote in message
...
ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe
so that it would be just under the fabric. The antenna is the type that
has a ceramic insulator on the bottom of the antenna that insulates the
stainless steel antenna from the airframe. Does the antenna need a large
plate for a ground plane or will a small plate be sufficient? If a
ground plane is required, can the copper foil tape be used on the inside
of the fabric as the ground plane?



Two comments. One is that the steel tube fuselage will be an adequate
ground plane if the attach plate is electrically connected to the fuselage
tubes.

Two, that wire whip antenna was just fine for the day in which we had 90
channel radios and didn't go much above 124 MHz.. Do you and your radio a
favor and get one of those fiberglass whips that have the word "broadband"
in their descriptor. You will be a much happier camper.



Please excuse my "piggybacking" your question, can anyone with an informed
opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea at
first glance...



Read it again. The antenna is meant for composite aircraft or metal tube
aircraft IN A FIBERGLASS WINGTIP. No VHF antenna will work worth a darn
inside of a steel tube fuselage. Google on "Faraday Cage" or "Screen Room"
to get a picture of what is happening.

Jim


  #10  
Old October 24th 06, 09:00 PM posted to rec.aviation.homebuilt
Morgans[_2_]
external usenet poster
 
Posts: 3,924
Default Aircraft antennas


"XCOM" wrote

Our experience is that anything inside a structure especially metal is NOT
going to work. As one other said... put the aerial on the outside where it's
meant to go.


With careful reading, I take that he is asking about putting JUST the ground
plate under the fabric, with the aerial on the outside, as you suggest.
--
Jim in NC

 




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